A conventional Digital Subscriber Line (DSL) leased from a telecommunications company uses a Time Division Multiplex (CDM) carrier or circuit, over which connection-oriented digital communication is practiced The span of cable or line extending between the telecommunications provider and the Customer Premise Equipment (CPE) is sometimes referred to in the art as a local loop. The first precursor to DSL technology was Integrated Services Digital Network (ISDN), which unlike pure DSL, transmits data as analog data over a telephone line but uses analog-to-digital (A/D) conversion for receive at the user end and digital-to-analog (D/A) conversion for send at the user end with an ISDN modem. Pure DSL does not require any data transformation between analog and digital. All transmissions over the phone line are digital and DSL cable modems are used.
Multiple DSL users are typically connected to a Digital Subscriber Line Access Multiplexer (DSLAM), which is in turn connected to a high-speed network backbone, typically an Asynchronous Transfer Mode (ATM) backbone. A DSLAM also de-multiplexes data traveling from the backbone to other geographically disparate destination loops or circuits. Other typical and emerging DSL technologies include Asymmetric Digital Subscriber Line (ADSL), Symmetric Digital Subscriber Line (SDSL), Very high-speed Digital Subscriber Line (VDSL), and High-bit rate Digital Subscriber Line (HDSL). Most typical home users are familiar with ADSL.
In a TDM connection-oriented network, fault testing a DSL segment or line is typically enabled by a mechanism known in the art as a smartjack. A smartjack is typically implemented in known art, for example, over an HDSL line using T1/DS1 protocols. T1/DS1 is a digital subscriber (DS) level and data framing specification for synchronous digital streams at a T1 transmission rate of 1,544,000 bits per second.
A smartjack is an input/output (I/O) device running logic, such as software, that is installed between the telecommunications switch (Telco) and a CPE switch or router in a local loop on the customer end. A typical smartjack is a box that has a Telco-side RX/TX port and a customer or CPE-side RX/TX port.
A smartjack is activated remotely by an administrator when link testing is to be performed. When in test mode the smartjack is caused to physically loop every bit received on the RX link of a port to the corresponding TX link of the same port. The data frames sent to the box are received back over the same line without any modifying of the structure. The loop-back testing technique is a standard and is well known in the art as a method of determining whether a detected network fault is somewhere in the transmission network or on the customer site of a connection-oriented link When the device is not in test mode, it remains in a transparent pass through mode where every bit received (RX) on one port is sent on through to the transmit line (TX) on the opposite port the box transmitting data as such bi-directionally.
While a smartjack of prior-art enables fault testing and isolation on a TDM connection-oriented data link, it cannot be used with newer packet-switched or connectionless network lines, for example those using Transfer Control Protocol/Internet Protocol (TCP/IP) including Ethernet network technologies. For example, if a data packet were transmitted to a prior-art smartjack installed in a connectionless link, and it were to be returned to the source station unchanged from the same port it would be rejected by the sending station as not being addressed to the sending station. This is because data packet headers (IP or Ethernet) must be addressed for source (sender) and destination (receiver) according to request/response protocols.
If the destination address header field of a data packet for send is set to the address of the sending station while at the sending station then the packet could not be sent. In other words, data loops are not permissible in a connectionless architecture. Attempting a software workaround to this problem is also not productive, because if the packet could be transmitted from the sending station having the destination address of the sending station then the software at the CPE switch or router could not properly forward the packet.
Therefore, what is clearly needed is a fault-testing device somewhat similar to a smartjack that can provide the same fault testing services using a point-to-point tunneling protocol for a packet-switched network line.